Thermosonic flip chip bonding is a key manufacturing process in electronic packaging in which a semiconductor chip is electrically connected to a substrate or to another chip directly with conductive bumps instead of using interconnections such as bonding wires in wire bonding, or tapes in tape automated bonding (“TAB”). A flip chip device is manufactured by first forming electrically conductive bumps onto a semiconductor chip, and thereafter flipping the chip that is formed with the conductive bumps for directly bonding the bumps onto equivalently placed bond pads on a substrate or on another chip. The electrically conductive bumps may comprise gold or solder.
The process in which the conductive bumps are attached to the bond pads utilizes an ultrasonic transducer for providing ultrasonic vibrational energy to the conductive bumps, thereby bonding the conductive bumps onto the bond pads by mutual friction between the surfaces.
FIGS. 1A and 1B are an isometric view and a front view respectively of a transducer 100 of the prior art. The transducer 100 comprises a substantially symmetrical polyhedral main body, such as a horn 102. The horn 102 includes two base ends 110, with one base end 110 at each side of the horn 102. The base ends 110 converge towards a narrower section in the middle of the horn 102 for mounting a bonding tool.
The middle section of the horn 102 further comprises a tapered hole for inserting a bonding tool to hold a chip to perform bonding. The bonding tool may be in the form of a cylindrical collet 104. The collet 104 is locked into position by a collet screw 106.
The collet 104 comprises a vacuum hole disposed along its longitudinal axis. The vacuum hole of the collet 104 is connected to an air passage and a vacuum tube 114 extending from one of the base ends 110 of the horn 102. Hence, when vacuum suction is applied through the vacuum tube 114 during the bonding process, the collet 104 will be able to hold a chip at the tip of the collet 104 using vacuum suction.
The transducer 100 further comprises ultrasonic generators 108 mounted at the base ends 110 of the horn 102. Each ultrasonic generator 108 includes a stack of piezoelectric elements axially connected and pre-loaded by a back plate 112 screwed into the horn 102 to secure the piezoelectric elements.
When the ultrasonic generators 108 are activated, they will generate ultrasonic vibrational energy that is transmitted from the generators 108 to the base ends 110 of the horn 102, through to the collet 104, and eventually to the tip of the collet 104. As such, the tip of the collet 104 will vibrate in accordance with a characteristic driving frequency and a corresponding amplitude of vibration.
The ultrasonic vibrational energy generated by the ultrasonic generators 108 will excite the transducer 100 such that the amplitude of vibration along the longitudinal axis of the transducer 100 exhibits characteristics of a standing waveform. FIG. 2 is a graph showing the varying amplitudes of vibration 120 along the transducer 100 of the prior art when ultrasonic vibrational energy is generated. In order to deliver a maximum amplitude of vibration for the bonding process, the tip of the collet 104 is preferably disposed at an anti-nodal position 122 of the standing waveform. This is at the middle of the horn 102 where the amplitude of vibration is at its maximum.
The horn 102 of the transducer 100 further comprises four L-shaped flange mounts 116, each disposed respectively at the four corners of the horn 102 for attaching the transducer 100 to a bonding apparatus (not shown) through the use of mounting holes 118 in the flange mounts 116. The flange mounts 116 are integrally connected to the horn 102 through thin flanges such that they project from the front and back of the horn 102, and so that a contact area between the flange mounts 116 and the main body of the horn 102 is minimal.
As shown in FIG. 2, the mounting holes 118 of the flange mounts 116 are located at nodal points on the transducer 100 where there is a minimum amplitude of vibration. This is to prevent the loss of ultrasonic vibrational energy through transmission of the same via the mounting interface to the bonding apparatus. The vibrational energy lost through the flange mounts 116 is therefore minimized and the ultrasonic vibrational energy may be effectively transmitted from the horn 102 to the collet 104. It would be appreciated that reduced vibrational energy at the tip of the collet 104 will affect the bonding between the conductive bumps and the bond pads, and hence affect the strength of the interconnections between the semiconductor components.
During the bonding process, high pressure and power are required to drive the transducer 100 for simultaneously bonding a number of conductive bumps onto the bond pads. The collet 104 of the transducer 100 therefore will experience an upward reaction force when pushing down onto a chip. FIG. 3 is a Finite Element Analysis (FEA) mesh diagram showing the effect of an upward reaction force 124 exerted on the collet 104 of the transducer 100 of the prior art. When the upward reaction force 124 is exerted on the tip of the collet 104, the tip will retract and be shifted to a new position at 104′.
The upward force 124 exerted on the collet 104 introduces further reaction forces 126 experienced at the flange mounts 116, and hence bending moments are induced on the horn 102. Since the L-shaped flange mounts 116 are rigidly connected to the body of the horn 102 by relatively thin flanges, the areas of the flange mounts 116 are bent significantly by the reaction forces 126. These deformation regions 128 are circled in FIG. 3, and they cause internal stresses within the lattice structure of the horn 102. In turn, this leads to reduced structural rigidity of the transducer 100 as a whole and higher compressive extension. Consequently, the risk of permanent deformation is high. This is likely to cause misalignment of the chip with respect to the bonding surface during the placement of a chip onto the corresponding bond pads, and thus affecting the ability to properly bond the conductive bumps to the bond pads.
Moreover, if the flange mounts 116 are deformed and are offset from the nodal positions 124 of the standing waveform, the transducer 100 can also be easily excited by undesirable resonance frequencies transmitted through the bonding apparatus 118. This is of major concern if the undesirable resonance frequencies are close to the operating resonance frequency of the transducer 100. When this occurs, these acoustic interference frequencies will introduce disturbances, such as noise, to the ultrasonic bonding operation, and thus affect the bonding quality.
Another disadvantage of the deformation of the flange mounts 116 is that some ultrasonic energy will be lost through the flange mounts 116, resulting in lesser vibrational energy transmitted to the collet 104, and a reduced amplitude of vibration at the tip of the collet 104. This affects the bonding strength between the conductive bumps and the bond pads. This problem is especially apparent in the reduction of the shear strength of a bonded chip, resulting in the chip shearing off from the bonding surface more easily.
FIG. 4 is a graph showing compressive extension 132 occurring at the tip of the collet 104 when the transducer 100 of the prior art is applied with a compressive forces 130. During the bonding operation cycle, high pressure and power are repeatedly applied on the transducer 100. Thus, the transducer 100 will experience compressive forces on the tip of the collet 104. As such, the tip of the collet 104 will retract in accordance with the compressive forces when the horn body is bent and deformed. The deformation of the horn body 102 may be measured by compressive extension at the tip of the collet 104. If the compressive extension of the collet 104 is within a range such that the internal stress in the horn 102 has not exceeded the elastic limit of the strength of the material, no permanent or plastic deformation will be observable in the horn 102 when the compressive forces are removed.
The data recorded shows that when a compressive force 130 of 10 kgf is applied to the collet 104 at 0.05 minutes (3 seconds) per cycle, the collet 104 retracts to a maximum compressive extension 134 of approximately 26 μm and oscillates with an amplitude of 13 μm. When the compressive forces 130 were removed at the end of the bonding process, a permanent deformation 136 of approximately 13 μm was observed to have set in at the tip of the collet 104. This irreversible plastic deformation in the horn 102 will degrade the performance of the transducer 100.
An alternative approach which may alleviate some of the aforesaid problems of the prior art is illustrated in U.S. Pat. No. 6,758,383 entitled “Transducer for a Bonding Apparatus”. This approach provides a transducer which includes a pair of connecting portions symmetrically located on opposite sides of a horn parallel to the longitudinal axial centre of a horn main body. The connecting portions have four symmetrically located mounting screw holes for attaching the transducer to a bonding apparatus. The connecting portions help to protect the structural rigidity of the transducer.
However, the problem with such a transducer design is that the mounting screw holes are formed on the connecting portions, and the connecting portions are rigidly mounted to the horn body such that the connecting portions are subject to deformation forces together with the horn body during bonding. As a result, the connecting portions are similarly bent together with the horn as in the prior art described previously when reaction forces act on the collet. Consequently, permanent plastic deformation could still occur due to pronounced bending of the connecting portions and the horn main body after the bonding process.
It would be advantageous to avoid some of the aforesaid disadvantages of the prior art by separating the mounting portion of a transducer assembly from rigid connection to the horn body so as to reduce the bending of the mounting portion together with the bending of the horn, which would in turn reduce interference with vibration of the horn as well as the risk of permanent deformation of the transducer.